Modern industries which require maximum strength from light weight construction materials have long sought low density structural materials with high temperature strength and low temperature ductility. Titanium aluminide alloys are of particular interest for these industries which employ low density and high strength alloys for use at elevated temperatures.
The microstructure of intermetallic alloys is known to affect the physical properties of the alloy. Artisans have utilized a number of different approaches in the Ti-Al system to produce microstructures composed of ordered intermetallic phases: namely, the .alpha..sub.2 -Ti.sub.3 Al and the .alpha..sub.2 -Ti.sub.3 Al+the .gamma.-TiAl. The combination of several properties (for example, high strength and ductility at elevated as well as room temperatures, fracture toughness and creep strength) is of particular interest in the .alpha..sub.2 and .gamma. trititanium aluminum alloys well known in the art. More contemporary titanium aluminides have fracture toughnesses which exceed those provided by the earlier .alpha..sub.2 and .gamma. titanium aluminides.
U.S. Pat. No. 4,292,077 discloses an alloy of the trititanium aluminum type (Ti.sub.3 Al) comprised of aluminum, niobium and titanium in relative atomic percent compositions of 25 to 27% Al, 12 to 15% Nb and the balance titanium. The patent discloses specific compositional ranges for the Ti.sub.3 AlNb alloys which are quite narrow. Furthermore, the physical properties are dependent upon the narrow percent composition of the alloys.
As knowledge of the titanium-aluminum alloy system expanded, a clearer understanding of the relationship between the composition of the alloy and microstructure developed. Therefore, the titanium-aluminum-niobium alloys based on homophase, orthorhombic, Ti.sub.2 AlNb, microstructure developed from the titanium aluminum alloys. These titanium aluminides are known to have a single phase, orthorhombic crystal structure unlike the ordered, hexagonal, DO.sub.19, structure of the earlier Ti.sub.3 Al alloy as discussed in "The Mechanical Properties of Titanium Aluminides Near Ti-25Al-25Nb", TMS Symposium on Microstructure/Property Relationships in Titanium Alloys and Titanium Aluminides, R. G. Rowe, Oct. 7, 1990. This Symposium paper discloses a titanium-aluminum-niobium alloy composed of a homophase, orthorhombic microstructure. However, although the disclosed titanium alloy had increased room temperature ductility and fracture toughness as compared with the traditional Ti.sub.3 Al alloy systems, it was shown to lose effective strength at high temperature.
U.S. Pat. No. 4,983,357 discloses a titanium-aluminum-niobium alloy with improved strength, specifically, a titanium aluminide alloy having excellent room temperature fracture toughness, high-temperature oxidation resistance and high-temperature strength as compared with previous titanium-aluminum alloys. The disclosed alloy contains from 29 to 35 wt. % of aluminum, 0.5 to 20 wt. % of niobium, at least one element selected from the group consisting of 0.12 to 1.8 wt. % silicon and 0.3 to 5.5 wt. % zirconium, the balance being titanium and incidental impurities. The disclosure further reveals that silicon and zirconium function to improve the high-temperature strength of the titanium aluminum alloy and that the absence of silicon and zirconium results in a titanium aluminide which does not have the desired physical characteristics.
A recent attempt to avoid additional impurities while providing superior physical properties in Ti-Al-Nb alloys is disclosed in U.S. Pat. No. 5,032,357. The patent discloses an alloy having from about 18 to 30 atomic percent aluminum, about 18 to 34 atomic percent niobium, with the balance being essentially titanium. The disclosed alloy has a superior combination of fracture toughness and high yield strength up to 1500.degree. F.--"superior combination" meaning that the alloy has at least as high or higher combined fracture toughness and yield strength than prior art trititanium aluminum alloys. Although the titanium aluminide alloys disclosed contain only the additional element niobium, niobium percentage ranges disclosed are above 16 atomic percent and have a microstructure comprised of the homogeneous orthorhombic phase - Ti.sub.2 AlNb.